I really don't know how to feel about this one. Gandalf is immortal and Dragons are mortal but I've always felt Dragons had more raw destructive power than anything else in Middle Earth. They were Morgoth's greatest creation and it was they who drove back the host of the Valar which contained not just the High Elf army of Valinor but also many Maia and were led by Eonwe. I always felt the weak spot, hollow on the breast, etc. was natures achilles hill to give way to stop something that in all other aspects would be unstoppable.